1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses of intermediate transfer type have been known in which toner images sequentially formed on a photosensitive member as an image carrier are sequentially superposed on an intermediate transfer belt as an intermediate transfer belt for intermediate transfer and these toner images on the intermediate transfer belt are then collectively transferred onto a transfer member for secondary transfer. In the case of a color image forming apparatus of this intermediate transfer type, toner images of different colors are transferred sequentially onto the intermediate transfer belt and superposed one on another, thereby forming a color toner image. Therefore, the toner images on the intermediate transfer belt have to pass through a primary transfer nip repeatedly. While the toner images are passing through the primary transfer nip repeatedly in this manner, so-called reverse transfer occurs, in which the toner is reversely charged and transferred onto the photosensitive member. With the occurrence of such reverse transfer, the toner of a solid image is partially decreased, which causes irregularity in the image.
To get around this problem, in one scheme, test pattern images of respective colors are formed on the intermediate transfer belt and the amount of adhered toner is detected for each test pattern image on the intermediate transfer belt after primary transfer of the test pattern images of all colors. Then, image parameters, such as a development bias for each color, are adjusted so that the amount of adhered toner for each color on the intermediate transfer belt after primary transfer of toner images of all colors is to be a predetermined amount. With this, the amount of adhered toner of a toner image for each color formed on the photosensitive member is increased by the amount of toner lost from the intermediate transfer belt due to reverse transfer. Therefore, even if the amount of adhered toner reduces due to reverse transfer, the amount of adhered toner for each color on the intermediate transfer belt after primary transfer of the toner images of all colors can be adjusted to the predetermined amount, which suppresses irregularity in an image. In this case, however, toner consumption increases, resulting in a higher cost for toner.
Japanese Patent No. 3344792 discloses a technology in which the toner on the intermediate transfer belt is charged again by a corona discharger before the toner on the intermediate transfer belt reaches the next primary transfer nip. With this, even if the toner on the intermediate transfer belt is reversely charged while passing through the primary transfer nip and the amount of charge decreases, the toner is charged again before reaching the next primary transfer nip. As a result, reverse charge of the toner on the intermediate transfer belt at the primary transfer nip can be suppressed, which prevents reverse transfer of the toner on the intermediate transfer belt at the primary transfer nip.
However, it is required to provide the corona discharger for charging again the toner on the intermediate transfer belt, which increases the cost, size, and power consumption of the apparatus. In particular, in the case of a tandem-type image forming apparatus including a plurality of photosensitive members, a corona discharger is provided at each portion between primary transfer nips, whereby increase in the cost, the size, and the power consumption is more significant.
Japanese Patent Application Laid-Open No. 2005-284275 discloses a technology in which the amount of adhered toner of a color (magenta M) to be first transferred onto the intermediate transfer belt after primary transfer of toner images of all colors is detected. If the amount of reverse transfer of the M-color toner exceeds a predetermined amount, a primary transfer bias (primary transfer current) for other colors (yellow Y, cyan C, and black Bk) is reduced by a predetermined value. In this manner, the second primary transfer bias onward is decreased, which suppresses the charging of the M-color toner on the intermediate transfer belt to reduce the amount of M-color toner of reverse charge. With this, the amount of M-color toner of reverse transfer can be reduced. Also, because an apparatus that charges the toner again, such as a corona discharger, is not used, it is possible to suppress an increase in cost and size of the apparatus. Furthermore, power consumption can be reduced, resulting in saving of energy.
However, in successive printing, if the primary transfer bias of the second color onward is reduced, primary transferability of the toner image of the second color onward is decreased after a predetermined number of printings, which causes an erroneous image with color unevenness.
The reason for this is explained below. At the primary transfer nip, a primary transfer bias having a polarity reverse to that of the toner is applied to the back surface of the intermediate transfer belt to form a primary transfer electric field. Therefore, when the intermediate transfer belt passes through the primary transfer nip, charges having the same polarity as that of the toner are moved onto the surface of the intermediate transfer belt due to an influence of the primary transfer electric field, and the charges having the polarity reverse to that of the toner are moved onto the back surface of the intermediate transfer belt. Thus, the surface of the belt is charged. If potential attenuation of the belt is not sufficient, the surface potential of the intermediate transfer member increased due to the primary transfer electric field cannot be attenuated by itself through the inside of the intermediate transfer belt even if the intermediate transfer belt rotates once after the toner image is transferred onto a transfer sheet for secondary transfer, and charges are left on the surface of the intermediate transfer member. As a result, when successive printing is performed, the potential of the intermediate transfer belt gradually increases. With an influence of the surface potential of the intermediate transfer belt, the primary transfer electric field acting on the transfer nip is weakened. As a result, for the second color onward in which the primary transfer bias is decreased to weaken the transfer electric field, the primary transfer electric field is further weakened. With this, primary transferability of the toner images of the second color onward decreases after a predetermined number of printings when successive printing is performed.
Moreover, if the potential attenuation of the belt is low, the potential history of the previous image is left on the surface of the intermediate transfer belt and a residual image of the toner at the time of the previous image formation occurs on the toner image transferred onto a recording medium for secondary transfer at the time of the next image formation.